It is well known that a public switched telephone network (PSTN) comprising T1 facilities can form a basis for a virtual digital network providing 64 kb/s channels. For example, by synchronizing a pulse code modulation (PCM) modem to an 8 kHz sampling rate provided in a central office and using 8-bit PCM words for data transmission, the modem can theoretically achieve a data rate up to 64 kb/s.
However, in practice, due to power constraints and such channel impairments as echo and intersymbol interference, the highest data rate achievable by the PCM modem is about 56 kb/s. This rate may be further reduced as the central office periodically "robs" the least significant bit (LSB) of the PCM words and substitutes it with a signaling bit. As is well known, the robbed bit signaling is necessary for indicating call statuses to effect call administration in the PSTN. While the robbed bit substitution does not cause significant distortion in voice communications, it causes significant degradation in the data communications because of the loss of transmitted bits occasioned thereby. In addition, the robbed bit substitution undermines proper training of an equalizer in the PCM modem, thereby adversely affecting channel equalization for the data communications.
The above problem caused by robbed bit signaling can be fully overcome using the technique disclosed in the co-pending, commonly assigned United States patent application of Y. Lai entitled "Technique for Effectively Treating Robbed Bit Signaling in Data Communications," Ser. No. 08/962,516, filed Oct. 31, 1997. Specifically, the disclosed technique can be used to detect two types of robbed bit signaling, namely, type A and type B robbed bit signaling, affecting data communications received by a PCM modem. The type(s) of the robbed bit signaling is identified during training of an equalizer in the modem, and the robbed bit signaling of the identified type is taken into account to properly train the equalizer. The robbed bit signaling type identification involves transmission of a four-level training signal through the PSTN to the modem. The signal thus received by the modem is processed by training circuitry therein which includes a level adapter. Based on the received signal, the level adapter utilizes a first mechanism to determine presence of any type A robbed bit signaling, followed by a second, different mechanism to determine presence of any type B robbed bit signaling.